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New instcombine rule: max(~a,~b) -> ~min(a, b)

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This case is interesting because ScalarEvolutionExpander lowers min(a,
b) as ~max(~a,~b).  I think the profitability heuristics can be made
more clever/aggressive, but this is a start.

Differential Revision: http://reviews.llvm.org/D7821



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230285 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjoy Das
2015-02-24 00:08:41 +00:00
parent 8d16a81c33
commit f922d9cfe4
4 changed files with 134 additions and 23 deletions
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33
lib/Transforms/InstCombine/InstCombineSelect.cpp
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@ -1145,12 +1145,14 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
if (Instruction *FoldI = FoldSelectIntoOp(SI, TrueVal, FalseVal))
return FoldI;
Value *LHS, *RHS, *LHS2, *RHS2;
SelectPatternFlavor SPF = MatchSelectPattern(&SI, LHS, RHS);
// MAX(MAX(a, b), a) -> MAX(a, b)
// MIN(MIN(a, b), a) -> MIN(a, b)
// MAX(MIN(a, b), a) -> a
// MIN(MAX(a, b), a) -> a
Value *LHS, *RHS, *LHS2, *RHS2;
if (SelectPatternFlavor SPF = MatchSelectPattern(&SI, LHS, RHS)) {
if (SPF) {
if (SelectPatternFlavor SPF2 = MatchSelectPattern(LHS, LHS2, RHS2))
if (Instruction *R = FoldSPFofSPF(cast<Instruction>(LHS),SPF2,LHS2,RHS2,
SI, SPF, RHS))
@ -1161,6 +1163,33 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
return R;
}
// MAX(~a, ~b) -> ~MIN(a, b)
if (SPF == SPF_SMAX || SPF == SPF_UMAX) {
if (IsFreeToInvert(LHS, LHS->hasNUses(2)) &&
IsFreeToInvert(RHS, RHS->hasNUses(2))) {
// This transform adds a xor operation and that extra cost needs to be
// justified. We look for simplifications that will result from
// applying this rule:
bool Profitable =
(LHS->hasNUses(2) && match(LHS, m_Not(m_Value()))) ||
(RHS->hasNUses(2) && match(RHS, m_Not(m_Value()))) ||
(SI.hasOneUse() && match(*SI.user_begin(), m_Not(m_Value())));
if (Profitable) {
Value *NewLHS = Builder->CreateNot(LHS);
Value *NewRHS = Builder->CreateNot(RHS);
Value *NewCmp = SPF == SPF_SMAX
? Builder->CreateICmpSLT(NewLHS, NewRHS)
: Builder->CreateICmpULT(NewLHS, NewRHS);
Value *NewSI =
Builder->CreateNot(Builder->CreateSelect(NewCmp, NewLHS, NewRHS));
return ReplaceInstUsesWith(SI, NewSI);
}
}
}
// TODO.
// ABS(-X) -> ABS(X)
}